The present invention relates to an oxide superconducting material and a process for producing said superconducting material. More particularly, the present invention relates to a REBa.sub.2 Cu.sub.3 O.sub.y oxide superconducting material (RE represents Y, Ce, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu or a combination of two or more of them), in which superconducting material RE consists of at least two elements selected from Y, Ce, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu, and which superconducting material gives a high critical current density (Jc) even in a highly magnetic field, as well as to a process for producing said superconducting material by subjecting raw material powders containing the at least two RE elements, to melting, slow cooling and heat treatment in this order under particular conditions.
Oxide superconducting materials have high critical temperatures and vigorous research is under way in order to put them into practical use. For example, Japanese Patent Application Laid-Open No. 291814/1988 proposes, as an oxide superconducting material, a superconducting composition containing rare earth elements (Y and Ce) in obtaining an oxide superconducting material as a bulk material, sintering has heretofore been used generally. The oxide superconducting materials obtained by sintering have such a microstructure that the grains are fine and a number of grain boundaries exist inside. In these oxide superconducting bulk materials obtained by sintering, the superconducting grains are combined with each other by a weak-link, making it impossible to obtain a high Jc.
Meanwhile, in superconducting materials of single crystal structure, it is known that no grain boundary problems as mentioned above exist and that a high Jc is obtained even in a highly magnetic field. Hence, it was investigated to allow the superconducting material of microstructure obtained by sintering to approximate a single crystal structure. Furthermore it was proposed to disperse particles of a non-superconducting phase in a single crystal phase to fix the magnetic flux line, that is, to form pinning centers. For example, melting processes represented by the MTG process (the Melt Textured Growth process) were proposed. In the MTG process, a rare earth oxide superconducting material, for example, is slowly cooled generally from the incongruent melting point of the 123 phase [REBa.sub.2 Cu.sub.3 O.sub.y (RE is a rare earth element including Y)] to give rise to a peritectic reaction between the 211 phase (RE.sub.2 BaCuO.sub.5) and the liquid phase to cause crystal growth; the 211 phase exists as particles inside the crystals because of an incomplete peritectic reaction after growth and provides pinning centers; as a result, the rare earth oxide superconducting material obtained shows a high Jc even in a magnetic field. The oxide superconducting material obtained by the MTG process, however, has various disadvantages in that the particles of the 211 phase are large and their distribution is non-uniform and that cracks exist along the direction of crystal growth.
Also, the QMG process (the Quench and Melt Growth process) was proposed in Japanese Patent Application Laid-Open No. 153803/1990, as an improved process for the MTG process. In the QMG process, raw materials for a rare earth oxide superconducting material are subjected to melting, quenching and solidification to obtain an intermediate comprising a Ba-Cu oxide phase and an Y.sub.2 O.sub.3 phase or the like of 50 .mu.m or less dispersed in said oxide phase, or Y.sub.2 O.sub.3 and a Ba-Cu oxide are mixed to obtain a plate-like material of 5 mm or less in thickness or a linear material; the intermediate or the plate-like or linear material is heated at the incongruent melting point of the 123 phase to convert to a semi-molten state and then is cooled slowly from that temperature at a given cooling rate to give rise to a peritectic reaction between the 211 phase and the liquid phase to grow a 123 phase in which a 211 phase of 20 .mu.m or less is finely and uniformly dispersed. According to the disclosure in the above document, the superconducting material obtained by the QMG process exhibits a very high pinning effect and gives an excellent Jc in a highly magnetic field. Further, the MPMG process (the Melt Powder and Melt Growth process) was proposed. In the MPMG process, a material obtained by melting, quenching and solidification according to the QMG process is ground for higher shapability and the obtained superconducting material is said to give the same high Jc as in the QMG process.
The QMG process and the MPMG process, as compared with the melting process, can exhibit a high pinning effect and give an excellent Jc, but require molding into a particular shape or two-stage melting (i.e. melting - quenching and solidification - semi-melting - slow cooling), making the procedure complex. Further, when the QMG process and the MPMG process are conducted via a step of a quenching and solidification to prevent the agglomeration of Y.sub.2 BaCuO.sub.5 phase to obtain a finely dispersed structure, a platinum crucible is requisite but the platinum may react with the rare earth oxide superconducting material and may invite deterioration of or variation in superconductivity of the material. Moreover, a means for enabling the high-rate quenching is required. Therefore, there is desired a process which can produce industrially in a simple procedure an oxide superconducting material exhibiting the same properties as those obtained in the QMG process or the MPMG process.